Surface treatment of metals



metal articles is in many cases limited practically UNITED STATES.

PATENT OFFICE arcane summon TREATMENT or names John 0. Redmond and Ralph w. mm, Youngstown. Ohi

No Drawing. Application September 16, 1938 Serial No. 230,210

9 Claims.

The present invention relates to the conditioning of the surface of metal articles which may contain oxide, scale, and the like or even appear apparently clean and unoxidized to the eye, by removing all the surface oxides and other contamination and imparting an improved type of metal surface preparatory to the application of a protective metal coating.

It is well known and recognized that the successfulness of protective coatings applied to by the ability of such coatings to withstand subsequent fabricating operations which are necessary before an object may be considered a finished product. In many cases, although possible, it is an economic disadvantage to apply protective coatings to metal articles after the last fabricating or forming operation has been performed, whereas, if a protective coating can be applied early in the process of fabrication while the material to be used is still in a continuous state, great savings can be effected. In fact, it is a common procedure in industry to test the quality of an applied metal coating by carrying out tests designed to gauge how much distortion a certain metal which has been coated with a protective metal will endure before the coating is torn from the base metal. It is also recognized that the protective value of an applied metal coating on a metal object is dependent to a large extent upon its adherence to the base metal. For this reason, any method of treating a surface so that it will bond, or adhere, in a superior manner to any applied protective coating is of extreme importance and of great economic value in the art.

It is well known that when ferrous and many non-ferrous metals are heated and exposed to the atmosphere, or merely subjected to atmospheric exposure for long periods without heating, the surface of the metal becomes oxidized and the article thus coated with a film comprising the oxides of the metal and sometimes'other compounds as well, forming a hard scale which is frequently firmly adherent and extremely difflcult toremove. Such scale and other surface contamination usually must be removed for proper performance of subsequent operations such as rolling, forming and the like, or applications of a protective coating of some other metal such as zinc or tin, or of enamel, paint, lacquer or the like, while in some instances best practice requires that the clean and descaled surface be etched to permit it to make a satisfactory bond with the subsequently applied coating material.

Numerous methods have heretofore been utilized for removing scale and other surface contamination from metal articles and oxide scale has generally been removed from ferrous articles by immersing the articles in a bath contain-' ing an agent, usually an acid, which reacts with the scale to produce a water soluble compound dissolving in the bath or in a subsequent washing water, leaving the surface of the metal free of the scale and its reaction product, some of the scale being dislodged mechanically from the surface through the action of the solution and the mechanical action of gaseous products of the reaction on the base metal. In the practice of, this method commonly lmown as pickling, a dilute solution of a suitable acid, for example, sulphuric or murlatic acid, is usually employed as the picking agent, and relatively large quantitles of it are required to maintain a bath of sufllclent volume to permit immersion of the articles to be pickled. Moreover, the method is a a slow one and involves much handling of the articles to be cleaned, thus entailing large expense for labor which, added to the cost of the pickling agent, makes it unduly expensive. Furthermore, if an ordinary etched surface on the :5

articles is desired for the purpose of obtaining a bonding in a subsequent metal coating operation, it can only be obtained by continued immersion in the pickling bath after the surface has been cleaned and descaled and apart from the indilute or concentrated forms, into heated retorts in the presence, or absence, of air with the object" in view of loosening thescale so that the articles may be cleaned more easily in a subsequent sand blast or abrading operation. The surface obtained by these methods is no more desirable than that obtained with the ordinary pickling practice from the standpoint of a subsequent pro tective metal coating operation.

Another suggested method refers to the removal of drawing compounds specifically hydrocarbons, mineral or petroleum oils, lubricants and the like, from clean metals which do not have oxidized surfaces, for the purpose of a subsequent coating operation. This method exposes the metal to be cleaned alternately to a water wash and then to the action of moist chlorine gas without the application of heat and is inoperable on dry metals or in the absence of moisture. Furthermore, although it may leave the metal in a cleaner state and consequently more suitable to receive a subsequent coating operation, it does not impart to the surface of the metal any qualities which might enable it to make a better bond to any coating subsequently applied.

In some instances the deoxidization treatment of metals, sometimes known as "bright annealing, has been practiced, involving heat treatment of the articles in the presence of a reducing agent such as finely divided carbon, hydrogen, coke oven gas and other reducing agents which, in the presence of heat, reduce the oxide of which the scale is formed to the unoxidized metal which leaves on the surface of the parent metal a fllm of the reduced metal in an undesirable physical state known as "sponge metal." Such a surface is not suitable to the reception of a. protective coating, does not receive the bright finish which may be imparted to clean metal by rolling and the like, and in general must be removed before any further operation on the article may be performed. The removal of this type of film from' the surface of metals can be accomplished only slowly by means of any of the known methods of cleaning or descaling, since it reacts'only slowly with most reagents practical for use in removing it.

Our invention is directed to improvements in the art of conditioning metal surfaces, without the employment of acids, or fluid baths containing corrosive reagents, and without the production of sponge. metal by chemical reduction of the contaminating material, by removing scale deposits formed of oxide or other undesirable compounds contaminating the surface of metal articles, if same are found on thesurface oi-the metal, and subjecting the surface of the base metal to a treatment which gives it an improved type of clean, etched or matted surface distinguishable from the ordinary etched surface obtained by the well known pickling methods in its ability to form a superior bond than heretofore known to a subsequently applied metal, or other protective coating. In fact, our method can be utilized to effect the removal of sponge iron metal as well as other contaminating substances at a rate of speed far in excess of those possible by other known methods.

In general it may be said that the present invention comprises subjectingthe articles to be treated to the action of hydrogen chloride gas at a certain minimum elevated temperature to convert the oxides and contaminating materials on the surface of the articles, as well as part of the base metal, to an anhydrous chloride of the base metal and water vapor, the lowest temperahire of operation being sufficient to cause the vaporization of said chloride with resultant removal of the chloride from the'surface of the metal; allowing the gas to react further on the surface thus freshly exposed, and then cooling the articles in the gas employed or in some other suitable non-oxidizing atmosphere.

Wehave discovered that oxide scale and sponge iron are removed fromv steel sheets, steel pipes" and the like in from five to ten seconds after introduction of hydrogen chloride, gas into .a chamber maintained at a temperature above 800 F. and wherein the articles are contained and that at this temperature the ferrous and ferric chlorides produced vaporize rather rapidly, leaving the articles relatively free from any sort of deposit and with a clean appearance similar to that obtained from pickling. On subsequently coating the articles treated in this manner with a zinc coating, that is to say, galvanizing the same,

we discovered that the adherence of the zinc coating was far greater than the adherence of a coating of zinc applied by the same method to steel articles which had been pickled in the ordinary manner.

It will be understood that the principal constituents of scale on ferrous metal articles are usually the ferrous and sesqui oxides (FeO and FezOa). We have discovered that the principal mechanism whereby this scale is removed from the surface of metals, according to our invention, is accomplished by the following fundamental reactions which take place.' These are of the type:

(Solid) (Gas) (Vapor) (Vapor) (1) Fe=O 6HC1 2FeCh 3H2O (2) Eco +2HCl- Feel: H2O

As is denoted by the reactions listed above, the scale reacts with the hydrogen chloride gas producing the ferrous and ferric chlorides of iron and water vapor. We have discovered that these reactions take place readily, rapidly, and efficiently between the temperatures of 800 F. and 1350 F., the optimum temperature being approximately 1100 F. Both the ferrous and ferric anhydrous chlorides of iron are readily vaporized and are formed in the vapor state at these temperatures. Thus the iron oxides are converted into gaseous chlorides and water vapor which leave the surface so that further action on the underlying metal or oxide may proceed. If the temperature is much lower, the chlorides do not vaporize and form a coating on the surface of 'only continue until a certain equilibrium mixture has been attained in which will be present hydrogen chloride gas, anhydrous ferric and ferrous chlorides of iron in the vapor state, and

- water vapor, all in their equilibrium concentrations at the temperature of operation. This is so because the chlorides which are in the vapor state, and consequently in intimate contact with the remainder of the atmosphere, soon reach this equilibrium concentration and stop the reaction.

Since water vapor is also a product of the reaction, the presence of water vapor in excess amounts at the start of the reaction slows the reaction down considerably, and brings the reaction to a stop at a different equilibrium mixture. The concentration of the chloride in this "mixture, or the amount of oxide converted into chloride-is very much less than it would have been if the system were substantially devoid of water vapor at the start of the reaction. In fact, if there is too much. moisture in the atmosphere at the start. no appreciable reaction will take place. This point may be demonstrated most -chloride gas.

easily by expressing the equilibrium constant for reaction (1) (Partial pressures P expressed in atmospheres) The equilibrium constant K is given by the product of the cube of the partial pressure of water vapor and the square of the partial pressure of ferric chloride vapor divided by the sixth power of the partial pressure of the hydrogen It has a definite value for a given temperature and the reaction will proceed in such a direction as to satisfy this value and will stop when this value has been reached. Thus we 4 see that if water vapor is present at the start, less chloride can be formed than if no water vapor were present at the start. of if a large amount of water vapor is present at the start, only an infinitesimal amount of chloride will be formed.'

Furthermore, we have found that these reactions are reversible reactions, and at temperatures higher than approximately 1350 F., the equilibrium constant assumes such a value that much less chloride is formed than at lower temperatures, that is, much more hydrogen chloride must be used for the formation of a given quantity of iron chloride vapor. For these reasons we have found that there is a definite upper limiting temperature at which the oxide can be economically converted into chlorides which is approximately 1350 F. whereas the lower limiting temperature of the process is the temperature of vaporization of the anhydrous ferrous and ferric chlorides of iron which is approximately with respect to the water vapor content, we have pointed out above that the presence of moisture 'at the start seriously impairs and hinders the speed of the reaction, even so far as to stop the reaction entirely if there is an overabundance of this substance. Thus while we preferably operate our process in the substantial absence of water vapor, the presence of water vapor having a partial pressure of approximately 15% of the total gas pressure can be tolerated without stopping the reaction, whereas if the water vapor at the start is substantially increased, say, for example, to 25%, the reaction will not take place. Furthermore, we have found, and it is quite evident from the above discussion, that our reactions proceed most rapidly when the hydrogen chloride gas used is substantially devoid of moisture at the start. Also, these reactions proceed at a veloeity which is extremely large compared to the velocities encountered in acid pickling baths where scale is removed by action of acids.

We have also found and it is evident from the foregoing discussion that in order to maintain a continued reaction, obtain the maximum efficiency of conversion of oxide into chloride per volume of hydrogen chloride gas expended, and

make use of the maximum rate of reaction, it is imperative to control the circulation of the gases in contact with the scaled objects in such a manner that the spent atmosphere is removed from contact with the articles as soon as the equilibrium mixture has been attained and immediately eliminated from the system.

V The above requirements exclude the' possibility of using acids in vaporized states, such as might be obtained by spraying, atomizlng, or boiling. For :xarnpie, it is not possible to use hydrochloric acid in carrying out our process. Hydrochloric acid is nothing more than hydrogen chloride gas dissolved in water. In its most concentrated form it contains approximately 65% water and 35% hydrogen chloride gas. When vaporized it is apparent that even the most concentrated hydrochloric acid would contain far too much water vapor to carry out the reactions necessary in our process. Hydrogen chloride gas may be compared to hydrochloric acid for our purpose in the same manner that chlorine would be compared to saturated chlorine water. Similarly other acids such as nitric acid and sulphuric acid, which are actually water solutions, cannot be used in carrying out our process. 1

We have found that chlorine gas itself cannot be utilized in the removal of scale from metal surfaces to produce articles which are clean and have a pickled appearance. The reasons for this may be twofold. First, it is questionable whether iron oxide reacts with chlorine at the temperatures in question without the presence of some reducing agent to take up the free oxygen which would necessarily be formed according to the reactions- (3), 2 Fez03+6 Cl2- 4 FeCh+3 0a (4) 2 Fe0+2 Ola- 2 FeCla-i-Oz In our method the hydrogen present in the hydrogen chloride reacts with the oxygen to form water vapor, whereas it is well known that even at the lowest temperatures of operation, e. g., those at which the chlorides formed would be vaporized, the chlorides of iron react with oxygen to form iron oxide and liberate chlorine. Furthermore, if the reactions did proceed towards the right at these temperatures, the oxygen liberated would be in contact with the metal surface and at the lowest temperatures necessary to vaporize the chloride, they would reoxidize the metal and form scale again. Thus we cannot use chlorine gas in practicing our process upon articles which are scaled to start with.

The foregoing discussion has been concerned primarily with the chemistry of scale removal from scaled objects since it is obvious that it is first necessary to expose the surface to the action of the gases employed in our process to conin so doing discuss simultaneously the conditioning of a surface which was originally scaled and of one which to start with was unsealed and clean.

when hydrogen chloride gas, substantially dry, is employed upon clean steel metals, the steel reacts with the hydrogen chloride gas with the formation of the chlorides of iron and the liberaes tion of hydrogen according to the reactionswe have also found that chlorine gas may be 1 substituted for hydrogen chloride gas in the prac tice of our method where the conditioning of scale' free surfaces is to be carried out, even though it cannot be utilized where scaled surfaces are concerned. Since in this case we do not have UI USS HBIUI BHUB to deal with oxides, the steel reacts with the chlorine gas to produce chlorides of iron without the liberation of oxygen as in the case of scaled surfaces, according to the reactions- (7) Fe+Clz FeCiz (8) 2 Fe+3 Ola- 2 FeCla Although these reactions (5, 6, 7, and 8) do not take place at ordinary temperatures, except in the presence of water, and then only very slowly (by means of the action of an acid formed by the water absorbing the chlorine or hydrogen chloride), they do occur on the application of heat. Beginning at about 400 F., they have an appreciable reaction velocity at 800 F. and are very rapid above that temperature in the absence of water vapor. However, the lower limit, as in the previously discussed case, at which our process can be carried out, is the temperature of the vaporization of the chlorides formed. Furthermore, these reactions do not continue until all the hydrogen chloride gas, or chlorine gas, has been converted into the chlorides of iron but as in the case of all gaseous reactions, continue only until a certain equilibrium mixture has been reached. Although water vapor is not a product of the reaction it is obvious from the previous' discussions that if water vapor is present in the system in substantial amounts, the water vapor will react with the iron chlorides and the equilibrium mixture then established will be the joint equilibrium mixture of two reactions. In'addition, at the temperatures of operation water vapor itself reacts with steel, converting the steel to the oxide and releasing hydrogen gas. All these considerations make it obvious that in the conditioning of metal surfaces the water vapor content must be kept as low as possible for best operation, whereas if the partial pressure of water vapor substantially exceeds 15% of the total gas pressure, we have found the desirable reactions are practically non-existent.

Therefore, the upper limiting temperature at which our process can be carried out for the conditioning of clean metal surfaces, whether hydrogen chloride gas or chlorine gas is employed, is entirely dependent upon the amount of water vapor present in the system. In the event that the water vapor is so controlled as to be present only in negligible quantities the upper limiting temperature of operation is to be chosen only with respect to other factors such as the metallurgical qualities of the metal, or the limitations of the containers, etc. If the water vapor content is of the order of 15%, the temperature of operation must belimited to approximately 1350 F. and if the water vapor content appreciably exceeds this amount the process cannot be carried out at all.

It has, therefore, been made clear that in the practice of our invention where we are concerned with metal objects containing oxides, scale, and other contamination upon their surfaces, we can employ hydrogen chloride gas, but not chlorine gas, to remove these substances and, if desired, also condition the exposed metal surface bycontinued treatment with hydrogen chloride gas, according to our process. Where we are concerned with clean metal objects, we may condition the surface by the use of either hydrogen chloride gas or chlorine gas in the manner described above.

We have found this method of treatment to have many advantages over the etching of metal by means of liquid acid solutions. First, the reactions take place much more rapidly. Whereas the time necessary to etch metals by means of acid baths is to be measured in periods of minutes and often in hours, our reactions are to be measured in seconds. For this reason, they have a unique advantage in the removal of the undesirable sponge iron from the surface of metals. Since the sponge iron is actually pure metal in an undesirable physical form, it canbe removed only very slowly by means of acid baths, whereas in the utilization of our method, sponge iron is removed as quickly as is the oxide of the metal. Second, although hydrogen gas is present in the system, the metal does not absorb the gas at the temperatures of operation and consequently there are no inclusions of hydrogen which often in the ordinary processes result in hydrogen embrittlement and in blisters. Thus a very annoying problem, which causes much waste in industry, is automatically eliminated. Third, we have discovered that metals conditioned in this examiner manner, although they do not present a much 1 different appearance to the eye than do ordinary etched metals, have a superior surface insofar as their ability to form a bond to any subsequently applied metal coating is concerned. Although we do not entirely understand the reason for the occurrence of this preferred type of surface, we believe that at the temperatures of operation, the gases employed by us have a twofold action upon the treated surface in addition to those mentioned. First, at the temperatures of operation the kinetic action of the gases attacking the steel is much more violent than in ordinary cases, and results in a surface which is more minutely uneven than an ordinary etched surface. Such a surface might be compared to an etched surface in the same way that the surface exposed by a pane of glass covered with an even coating of sand might be compared to the glass pane itself. The uneven surface would provide a much larger area for gripp the protective coating than would a comparatively flat surface for the same included area of the metal and thus give much greater strength to the bond of the two metals. For example, a completely flat surface one inch square would providea surface of one square inch for gripping, whereas if the surface were minutely uneven it might provide twice or even three times that area for gripping. Furthermore, it is believed that our process provides for the removal of other surface impurities which might be injurious to the adherence of a subsequently applied metal coating and which are not removed at ordinary temperatures. In other words, a more chemically clean surface is obtained than with the common methods now in. use. We have found that when a zinc coating is applied by the method of galvanizing to a steel surface conditioned according to our invention, the coating will suffer more distortion than will the base metal itself, that is, the base metal will be torn apart before the protective coating is severed from the base metal.

In the practice of our invention we do not limit ourselves to the use of pure hydrogen chloride or chlorine gases. Thus we may dilute the active gases by mixing them with some inert gas such as, for example, nitrogen, or a mixture of gases which do not enter into chemical reaction with the active gases or the metal surface at the temperatures of operation such as, for example deoxidizing atmosphere. In fact, we prefs. m practice our method when the active gas has i chloride content, or chlorine if it is being used, is between thirty and fifty per cent of the total mixture. Or we may vary the amount of dilution according to the requirements of a particular operation.

Of course, to prevent the growth of further scale on the article after completion of the chemical reactions, it is necessary that the cooling of the articles to room temperature take place under circumstances excluding oxygen or other oxidizing gases from their surfaces. This may be accomplished by keeping the articles enveloped by the active gases or inert gases during the operation of cooling.

It will be understood that the extent to which the several reactions proceed is dependent upon the time allowed and that after suflicient time has elapsed for substantial completion of the reaction between the hydrogen chloride gas and the scale or other sm'face contamination, the articles may be removed. They may be left in the chamber in contact with the active gas for a further period if it is desired that their surfaces be conditioned, the extent of conditioning being determined by the control of the time allowed therefor and of the temperature and of the dilution of the hydrogen chloride gas in the active atmosphere.

No specific reference has been made to the particular type of chamber for use in carrying out the method, as any chamber suitable for the purpose may be employed, but it is frequently of advantage to utilize a continuous furnace when conditioning ferrous articles preparatory to galvaniz ing, as this process may then form a step in the galvanizing operation. In some instances, however, it may be preferable to carry out the process as a batch method of operation in a suitable heating furnace so equipped as to enable the articles to be cooled afterconditioning without removing them from the furnace. When pipes are being cleaned, means may be provided for passing a stream of the active gas through their interiors so as to condition their inside as well as their outside surfaces. 7

Our invention may also be carried out in combination with heat treatment such as normalizing or box annealing through the introduction of the active gases into the normalizing furnace or annealing box at a suitable point during the course of operation.

This application is a continuation in part of our application filed August 9, 1937, Serial No. 158,153.

What is claimed as new is:

1. A method of conditioning a ferrous bodyvolatilize, in a non-oxidizing atmosphere comprising'a gas selected from the group consisting of chlorine and hydrogen chloride, and having a moisture content low enough to permit iron chlorides to be formed at said temperature.

3. A method of conditioning an unscaled ferrous metal that comprises subjecting it to a temperature great enough to cause iron chlorides to volatilize, in a substantially anhydrous, nonoxidizing atmosphere comprising a gas selected from the group consisting of chlorine and hydrogen chloride.

4. A method of conditioning an unscaled ferrous metal that comprises subjecting it to a temperature great enough to cause iron chlorides to volatilize, in a non-oxidizing atmosphere comprising chlorine, and having a moisture content low enough to permit iron chlorides to be formed at said temperature, and then cooling it in a nonoxidizing atmosphere.

5. A method of conditioning a scaled ferrous metal that comprises subjecting it to a temperature from about 800 F. to about 1350 F., in a non-oxidizing atmosphere comprising hydrogen chloride and having a moisture content low enough to permit iron chlorides to be formed a said temperature.

6. A method of conditioning a scaled ferrous metal that comprises subjecting it to a temperature from about 800 F. to about 1350 F., in a non-oxidizing atmosphere comprising hydrogen chloride. and further subjecting the exposed metal surface to a temperature great enough to cause iron chlorides to volatilize, in a non-oxidizing atmosphere comprising hydrogen chloride, said atmospheres having a moisture content low enough to permit iron chlorides to be formed at said temperatures.

7. A method of conditioning a scaled ferrous metal that comprises subjecting it to a temperature from about 800 F. to about 1350 F., in a non-oxidizing atmosphere comprising hydrogen chloride and having a moisture content low enough to permit iron chlorides to be formed at said temperature, and then cooling it in a nonoxidizing atmosphere.

8. A method of conditioning a ferrous body that comprises subjecting it to a temperature great enough to cause iron chlorides to volatilize, in a non-oxidizing atmosphere comprising a gas that reacts with the surface of said body to form iron chlorides and having a moisture content low enough to permit iron chlorides to be formed at said temperature.

9. A method of conditioning a ferrous body in a closed space that comprises passing into said space a gas capable of reacting with the surface of said body to form iron chlorides, while said sur- 

